This invention relates to electronic oscillator circuits, and more particularly, to differential voltage controlled oscillator circuits.
In a tuned oscillator, the frequency of oscillation may be varied by means of a control signal. The term voltage controlled oscillator (hereinafter referred to as VCO) generally refers to a class of oscillators whose frequency is a function of the voltage of the input signal. Thus, conventional VCOs are constructed to generate an output signal having a frequency that is a function of the voltage of the input signal. VCOs are used in a wide variety of applications, and are particularly useful in transmitters and receivers to generate an oscillation at a specific frequency.
A conventional differential controlled oscillator 1 topology is shown in FIG. 1. This conventional differential VCO 1 topology includes biasing circuitry 2, an oscillator core 4, a tuning circuit 6, inductors 8a, 8b and output capacitors 10a, 10b. 
The biasing circuitry generally provides the voltage drop that serves as the bias for core transistors 18, 20. The biasing circuitry comprises a bias resistor 12 or optionally a current source that is connected between ground 30 and the emitters of the core transistors 18, 20, and two bias resistors 14, 16 that are connected between the bases of the core transistors 18, 20. The biasing resistor 12 also provides some common mode rejection to the differential pair of the core transistors 18, 20. The two bias resistors 14, 16 provide a voltage input at the bases 18b, 20b of the core transistors 18, 20, respectively.
The oscillator core 4 may comprise, for example, at least one pair of bipolar junction transistors 18, 20 (referred to above as core transistors), a first pair of capacitors 22, 24, a capacitor 26 cross-coupled between the base 18b of core transistor 18 and the collector 20c of core transistor 20, and a second capacitor 28 cross-coupled between the base 20b of core transistor 20 and the collector 18c of core transistor 18. The two capacitors 22, 24 are connected between the bases of bipolar junction transistors 18, 20 and the AC ground node 30, respectively. Two cross-coupled linear capacitors 26, 28 provide a positive feedback loop to produce oscillations. The ratio between the two linear capacitors 26, 28 to the capacitors 22, 24, respectively sets the amount of positive feedback in the oscillator. The core transistors 18, 20 provide gain in the VCO 1, and emitters 18e and 20e of each core transistor 18, 20 are coupled to an AC ground node 30 by bias resistor 12.
The VCO 1 essentially functions as a signal generator that generates oscillations at a specific oscillation frequency (w). A tuning circuit 6 may comprise, for example, a pair of varactors 34a, 34b and a pair of bias resistors 36a, 36b connected in parallel across the varactors 34a, 34b. A capacitor 38ais coupled to the varactor 34a, and is connected between the collector 18c and the bias resistor 36a. Another capacitor 38b is coupled to the varactor 34b, and is connected between the collector 20c and the bias resistor 36b. The inductors 8a, 8b bias the collectors 18c, 20c of the core transistors 18, 20, and have some influence over the loop gain. Inductors 8a, 8b combine with the tuning circuit 6 create a variable tank circuit in the VCO 1. In other words, the inductors 8a, 8b, along with the total capacitance of the oscillator core 4, may be used to set the oscillation frequency of the VCO 1 which is   w  =            1              LC              .  
The inductance (L) of the VCO is determined primarily by the values of the inductors 8a, 8b, while the capacitance (C) of the VCO is all of the capacitance in the VCO 1. For example, the values chosen for the linear capacitors 26, 28 influence the oscillation frequency (w) of the VCO 1 since those linear capacitors are placed across the nodes of the tank circuit. Importantly, the total capacitance (C) of the VCO 1 can be varied via the tuning circuit 6 since each varactor 34a, 34b in the tuning circuit 6 can be characterized by a voltage-sensitive capacitance. The control voltage Vtune 32 may be used to change the voltage across these varactors 34a, 34b, which in turn changes the capacitance of the varactors 34a, 34b to thereby change the total capacitance of the VCO 1. Thus, by changing the control voltage Vtune 32, the oscillation frequency (w) of the VCO 1 is varied.
The output coupling capacitors 10a, 10b are connected between the output of the VCO 1 and the output terminals 40a, 40b of the VCO 1. The capacitors 10a, 10b couple the output signal to the next circuit, and determine how much signal will be coupled to the output. The total output signal is the difference between output A 40a and output B 40b or vice versa.
The performance of an oscillator may be characterized by a number of attributes, including spectral purity, frequency stability, and signal-to-noise ratio. To achieve frequency stability it is desirable to reduce the amount of phase noise. One problem that typically occurs in the VCO shown in FIG. 1 is that the output may exhibit an unacceptable level of phase noise which decreases the signal-to-noise ratio of the VCO.
The signal-to-noise ratio may be increased by increasing current in the VCO to increase the amount of signal with respect to phase noise. However, a 6 dB improvement in phase noise typically requires doubling of current in the VCO. Such an increase in current consumption is not an option in many applications. Accordingly, there is a need in the art for an improved VCO with reduced phase noise that may be implemented in a simple, inexpensive circuit. It would be highly desirable to reduce phase noise without an appreciable increase in current consumption. It is also highly desirable to accomplish both of these objectives without substantially increasing the cost or size of the VCO.
Differential voltage controlled oscillators are provided including a differential oscillator core, a first linear capacitor, a second linear capacitor, a first phase noise reduction device, and a second phase noise reduction device. The differential oscillator core has a first output that is capacitively coupled to a first differential input thereof by a first semiconductor junction having a first non-linear capacitance, and a second output that is capacitively coupled to a second differential input thereof by a second semiconductor junction having a second non-linear capacitance. The first linear capacitor is electrically cross-coupled from the first differential input to the second output, while the second linear capacitor electrically cross-coupled from the second differential input to the first output. The cross-coupled first and second linear capacitors provide a positive feedback loop to generate oscillations. The ratio between the first and second linear capacitors sets the amount of positive feedback in the oscillator. The first phase noise reduction device has a first non-linear capacitance characteristic, and is electrically cross-coupled from the first differential input to the second output. The second phase noise reduction device has a second non-linear capacitance characteristic, and is electrically cross-coupled from the second differential input to the first output. By using the first and second phase noise reduction devices to add non-linear junction capacitance to each side of the differential structure, the non-linear junction capacitances of the first and second semiconductor junctions are effectively canceled out to reduce the phase noise of the oscillator.
The first and second phase noise reduction devices are preferably p-n junction devices having non-linear capacitance characteristics that are preferably matched to the first and second non-linear capacitances over the entire oscillation cycle of the oscillator. For example, the first and second phase noise reduction devices may comprise first and second open-emitter bipolar transistors, respectively. In this situation, a non-linear capacitance of a collector-base junction of the first open-emitter bipolar transistor is preferably matched to the second semiconductor junction, while a non-linear capacitance of a collector-base junction of the second open-emitter bipolar transistor is preferably matched to the first semiconductor junction. The first semiconductor junction may comprise a collector-base junction of a first bipolar input transistor, while the second semiconductor junction may comprise a collector-base junction of a second bipolar input transistor. The first and second bipolar input transistors are also preferably matched.
According to another preferred embodiment, a differential voltage controlled oscillator circuit is provided for generating a low phase noise, differential signal. This differential voltage controlled oscillator circuit comprises means for generating oscillations including first and second oscillator output, a first input terminal capacitively coupled to the second oscillator output, a second input terminal capacitively coupled to the first oscillator output, first means for reducing phase noise, and second means for reducing phase noise. A tuning circuit may be connected across the first and second oscillator output terminals, and a tank circuit may be connected in parallel with the tuning circuit between the first and second oscillator output terminals of the voltage controlled oscillator. A first linear capacitor may be electrically cross-coupled from the first input terminal to the second output, while a second linear capacitor may be electrically cross-coupled from the second input terminal to the first output. The first means for reducing phase noise is connected between the first input terminal and the second oscillator output, while the second means for reducing phase noise is connected between the second input terminal and the first oscillator output. The first and second means for reducing phase noise preferably comprise p-n junction devices, for example, first and second open-emitter bipolar junction transistors. The means for generating oscillations preferably includes first and second core transistors of a differential pair of transistors. The collectors of the first and second bipolar junction transistors may be cross-coupled to collectors of the second and first core transistors of a differential pair of transistors, respectively. The bases of the first and second bipolar junction transistors are preferably coupled to bases of the first and second core differential transistors of the differential pair of transistors, respectively. Preferably, the first and second core transistors of the differential pair of transistors are substantially identical to the first and second bipolar junction transistors.
Accordingly, voltage controlled oscillators (VCOs) according to the present invention exhibit reduced phase noise and/or reduced current consumption due to reduction and/or cancellation of non-linear junction capacitance. This may be accomplished by adding a transistor in parallel with each of the main transistors in the oscillator core. By adding non-linear junction capacitance in this manner to the differential structure, non-linear junction capacitance of the collector-base is effectively canceled out to thereby reduce the phase noise of the oscillator since there is less capacitance change across the differential output nodes in the VCO. As a result, reduction in phase noise is achieved without an appreciable increase in current consumption. Moreover, the reduction in phase noise is accomplished by adding a small number of extra components to the VCO circuit, thereby conserving valuable space and production cost.